The present invention relates to an iron compound catalyst for inhibiting the generation of dioxin and an incineration process of a municipal solid waste using the catalyst, and more particularly, to an iron compound catalyst for inhibiting the generation of dioxin, which enables complete combustion of a municipal solid waste and decomposition of dioxin precursors even at a low combustion temperature in intermittently operated municipal solid waste incinerators such as mechanical batch incinerators or semi-continuous incinerators, and which can inhibit the generation of dioxin due to a memory effect upon low-temperature combustion at the start-up or shut-down of the incinerators, without large-scale incinerator renovation or plant and equipment investment, and a incineration process of municipal solid waste using such an iron compound catalyst.
Upon waste disposal, various municipal solid wastes have been divided into reusable ones and the others. After the reusable ones are recovered as effective resources, remaining combustible municipal solid wastes have been usually incinerated. Municipal solid waste incinerators used for waste disposal are classified into four types according to its operation time a day (i.e., into mechanical batch incinerators, fixed batch incinerators, semi-continuous incinerators and full-continuous incinerators).
In large-scale continuously operated incinerators, incineration atmosphere and incineration temperature is maintained in a steady state, so that the incineration can be continuously conducted at a high temperature, resulting in complete combustion of municipal solid wastes and even heat-decomposition of chlorine compounds. Therefore, in such continuously operated incinerators, although chlorine gas or hydrogen chloride gas is generated, the amount of extremely harmful dioxin generated is relatively small. The dioxin is undecomposable in natural environment, and, therefore, when being taken into human bodies through water, the dioxin is accumulated within the human bodies, thereby causing a problem concerning a high carcinogenesis.
On the other hand, most of the intermittently operated incinerators have been operated only when a certain amount of municipal solid wastes is gathered or at predetermined time intervals. Accordingly, it takes several hours from the start-up until the incinerator can be operated in a safe and steady state. At every start-up operation, there is caused a low-temperature combustion condition in which the dioxin tends to be generated. Also, when the incinerator is stopped, a part of the municipal solid wastes is continued to smoke while causing incomplete combustion thereof. In such a case, when the incinerator is started up again, unburned substances caused due to incomplete combustion upon the shut-down of the incinerator on the previous day and upon the start-up thereof on the next day, still remain inside a flue of the incinerator or a dust collector. In consequence, there has been pointed out such a disadvantage that such a so-called memory effect that dioxin is re-synthesized and then discharged into an exhaust gas even though the temperature of the exhaust gas is as low as not more than 200.degree. C., is caused and continued for several hours after the start-up of the incinerator (refer to Kawakami, Mtsuzawa and Tanaka, "Lecture Papers of the 5th Meeting of Japan Waste Matter Institute", p. 264 (1994)).
At the present time, the generation of dioxin from such intermittently operated incinerators has become a social problem.
Therefore, it has been strongly desired to provide a catalyst for inhibiting the generation of dioxin, which enables complete combustion of municipal solid wastes and decomposition of dioxin precursors in the existing mechanical batch incinerators or semi-continuous incinerators, and can avoid the generation of dioxin due to the memory effect upon low-temperature combustion at the start-up or shut-down of these incinerators, without a large-scale incinerator renovation or plant and equipment investment, and an incineration process of municipal solid waste using such a catalyst which enables complete combustion of municipal solid wastes and decomposition of dioxin precursors in the existing mechanical batch incinerators or semi-continuous incinerators, and can avoid the generation of dioxin due to the memory effect upon low-temperature combustion at the start-up or shut-down of these incinerators.
Hitherto, as the prior arts relating to absorption or decomposition of extremely poisonous dioxin, various methods have been reported. For example, there are known a method of adsorbing harmful components such as dioxin with a cement-containing adsorbent after cooling an exhaust gas discharged from a municipal solid waste incinerator, separating and recovering dusts collected together with the adsorbent, and then kneading and solidifying a mixture of dusts and adsorbent recovered (Japanese Patent Application Laid-Open (KOAKI) No. 4-371714(1992)); a method of decomposing poly-halogenated aromatic compounds having at least five carbon atoms by heating at 200 to 550.degree. C. in the presence of a catalyst such as iron oxide (Japanese Patent Publication (KOKOKU) No. 6-38863(1994)); a method of removing halogenated aromatic compounds or the like from an exhaust gas or reducing amounts thereof by heat-treating at 300 to 700.degree. C. in the presence of a catalyst containing iron oxide (Japanese Patent Application Laid-Open (KOAKI) No. 2-280816(1990)); or the like.
In addition, there is known a method of preliminarily mixing combustible wastes with iron oxide or the like and burning the combustible wastes at a temperature of not less than 850.degree. C. under the coexistence of calcium compounds, iron oxide particles or the like (Japanese Patent Application Laid-Open (KOAKI) No. 8-270924(1996)).
Further, there is known a method of burning wastes in an incinerator under the coexistence of ferric iron oxide hydroxide particles or iron oxide particles containing sulfur and sodium in not more than predetermined amounts (Japanese Patent Application Laid-Open (KOAKI) No. 9-89228(1997)).
However, although it has been desired to provide an iron oxide catalyst for inhibiting the generation of dioxin, which enables complete combustion of municipal solid wastes and decomposition of dioxin precursors in exiting intermittently operated incinerators such as mechanical batch incinerators or semi-continuous incinerators, and can prevent the generation of dioxin due to the memory effect upon low-temperature combustion at the start-up or shut-down of these incinerators, and a incineration process of a municipal solid waste using such a catalyst, the methods described in the specifications of the above Japanese KOKAIs or KOKOKU are still unsatisfactory.
Namely, in the method described in Japanese Patent Application Laid-Open (KOAKI) No. 4-371714(1992), dioxin generated, etc., are adsorbed onto the surface of porous cement at the bag filter portion. In this method, the dioxin is merely transferred from an exhaust gas to the cement. Therefore, this method fails to essentially inhibit the generation of dioxin. Further, it is necessary to conduct a treatment for converting the adsorbed dioxin into unharmful substances.
In the method described in Japanese Patent Publication (KOKOKU) No. 6-38863(1994), poly-halogenated cycloalkyl compounds and poly-halogenated aromatic compounds in fly ash generated in an incinerator are decomposed by catalysts such as iron oxide, calcium carbonate or sodium carbonate in a fixed bed. However, huge plant and equipment investment is required to construct a facility for converting the fly ash into unharmful substances, at a rear stage of the intermittently operated incinerator. Therefore, such a construction is almost impossible practically.
In particular, in the intermittently operated incinerators such as mechanical batch incinerators or semi-continuous incinerators, the start-up and shut-down operations have been inevitably repeated. Therefore, incomplete combustion tends to be caused due to low-temperature combustion upon the start-up and shut-down of the incinerators. In this case, it is known that dioxin tends to be generated due to the memory effect from dioxin precursors contained in unburned substances which are adhered onto flue or dust collector upon the shut-down operation of the previous day.
It is difficult to sufficiently remove dioxin by the conventional method of removing dioxin generated after passing through the dust collector (Japanese Patent Application Laid-Open (KOAKI) No. 6-38863(1994)) or the like. Further, in the incineration method of pre-mixing municipal solid wastes with iron oxide particles, etc., (Japanese Patent Application Laid-Open (KOAKI) No. 8-270924(1996)), in view of the structure of the incinerator used therein, it is difficult to inhibit the generation of dioxin at the flue or dust collector due to the memory effect upon low-temperature combustion at the start-up or shut-down of the incinerator.
Also, in the incineration method using iron oxide hydroxide particles or iron oxide particles containing sulfur or sodium in not more than a predetermined amount (Japanese Patent Application Laid-Open (KOAKI) No. 9-89228(1997)), a sufficient catalytic activity of the iron oxide hydroxide particles or iron oxide particles can be exhibited at a high temperature at which the incinerator is operated at a steady state. However, under the low-temperature combustion condition upon the start-up of the intermittently operated incinerator, the conversion percentage of carbon monoxide into carbon dioxide at 250.degree. C. is disadvantageously low, as shown in Comparative Example 1 hereinafter. As a result of the incineration test using such particles, the generation of dioxin due to the memory effect upon low-temperature combustion cannot be sufficiently inhibited, as shown in Comparative Example 3 hereinafter.
Thus, it is difficult to directly evaluate the dioxin-inhibiting effect by measuring the concentration of dioxin in the exhaust gas. Under this circumstance, in the course of studies concerning specific catalytic activities for obtaining an alternate method of evaluating the dioxin-inhibiting effect, it has been found that a close interrelation exists between the conversion percentage of carbon monoxide into carbon dioxide and the decomposition reaction of dioxin precursors. As a result, it has been found that the dioxin-inhibiting effect can be evaluated by the conversion percentage of carbon monoxide into carbon dioxide.
Further, it has been found that the generation of dioxin in the combustion chamber of the intermittently operated incinerator, can be inhibited by spray-adding an iron compound catalyst composed of iron oxide particles or iron oxide hydroxide particles, which is capable of exhibiting an excellent specific catalytic activity even under the low-temperature combustion condition, to the combustion chamber, and in addition, when these particles are retained in the flue or dust collector together with fly ash, it is possible to achieve not only complete combustion of municipal solid wastes but also decomposition of dioxin precursors contained in unburned substances remaining in the flue and dust collector, even under the low-temperature combustion condition upon the start-up or shut-down of the incinerator, thereby inhibiting the generation of dioxin due to the memory effect.
Furthermore, it has been found that by spray-adding specific iron oxide particles and/or specific iron oxide hydroxide particles having an average particle size of 0.01 to 2.0 .mu.m and exhibiting a specific catalytic activity capable of converting carbon monoxide into carbon dioxide, as a catalyst for inhibiting generation of dioxin, in a specific amount into a combustion chamber of the intermittently operated incinerator by an air carrying method, it becomes possible to inhibit the generation of dioxin upon the low-temperature combustion. The present invention has been attained on the basis of the finding.